Silver halide photography has benefitted in this decade from the development of tabular grain emulsions. As employed herein the term "tabular grain emulsion" designates any emulsion in which at least 50 percent of the total grain proJected area is accounted for by tabular grains. Whereas tabular grains have long been recognized to exist to some degree in conventional emulsions, only recently has the photographically advantageous role of the tabular grain shape been appreciated.
Tabular grain emulsions exhibiting particularly advantageous photographic properties include (i) high aspect ratio tabular grain silver halide emulsions and (ii) thin, intermediate aspect ratio tabular grain silver halide emulsions. High aspect ratio tabular grain emulsions are those in which the tabular grains exhibit an average aspect ratio of greater than 8:1. Thin, intermediate aspect ratio tabular grain emulsions are those in which the tabular grain emulsions of a thickness of less than 0.2 .mu.m have an average aspect ratio in the range of from 5:1 to 8:1.
The common feature of high aspect ratio and thin, intermediate aspect ratio tabular grain emulsions, hereinafter collectively referred to as "recent tabular grain emulsions", is that tabular grain thickness is reduced in relation to the equivalent circular diameter of the tabular grains. Most of the recent tabular grain emulsions can be differentiated from those known in the art for many years by the following relationship: EQU ECD/t.sup.2 &gt;25 (1)
where
ECD is the average equivalent circular diameter of the tabular grains and PA0 t is the average thickness of the tabular grains. PA0 AR is the average tabular grain aspect ratio and PA0 ECD and t are as previously defined, PA0 ECD is the average equivalent circular diameter in .mu.m of the tabular grains and PA0 t is the average thickness in .mu.m of the tabular grains.
The term "equivalent circular diameter" is employed in its art recognized sense to indicate the diameter of a circle having an area equal to that of the projected area of a grain, in this instance a tabular grain. In keeping with the established practice in the art, both ECD and t are measured in micrometers (.mu.m). All tabular grain averages referred to are to be understood to be number averages, except as otherwise indicated.
Since the average aspect ratio of a tabular grain emulsion satisfies relationship (2): EQU AR=ECD/t (2)
where
it is apparent that relationship (1) can be alternatively written as relationship (3): EQU AR/t&gt;25 (3)
Relationship (3) makes plain the importance of both average aspect ratios and average thicknesses of tabular grains in arriving at preferred tabular grain emulsions having the most desirable photographic properties.
The following illustrate recent tabular grain emulsions satisfying relationships (1) and (3):
______________________________________ R-1 U.S. Pat. No. 4,386,156, Mignot; R-2 U.S. Pat. No. 4,399,215, Wey; R-3 U.S. Pat. No. 4,400,463, Maskasky; R-4 U.S. Pat. No. 4,414,304, Dickerson; R-5 U.S. Pat. No. 4,414,306, Wey et al; R-6 U.S. Pat. No. 4,414,310, Daubendiek et al; R-7 U.S. Pat. No. 4,425,425, Abbott et al; R-8 U.S. Pat. No. 4,425,426, Abbott et al; R-9 U.S. Pat. No. 4,433,048, Solberg et al; R-10 U.S. Pat. No. 4,434,226, Wilgus et al; R-11 U.S. Pat. No. 4,435,499, Reeves; R-12 U.S. Pat. No. 4,435,501, Maskasky; R-13 U.S. Pat. No. 4,439,520, Kofron et al; R-14 U.S. Pat. No. 4,478,929, Jones et al; R-15 U.S. Pat. No. 4,504,570, Evans et al; R-16 U.S. Pat. No. 4,520,098, Dickerson; R-17 U.S. Pat. No. 4,643,966, Maskasky; R-18 U.S. Pat. No. 4,656,122, Sowinski et al; R-19 U.S. Pat. No. 4,672,027, Daubendiek et al; R-20 U.S. Pat. No. 4,684,607, Maskasky; R-21 U.S. Pat. No. 4,693,964, Daubendiek et al; R-22 U.S. Pat. No. 4,713,320, Masasky; and R-23 U.S. Pat. No. 4,713,323, Masasky. ______________________________________
All of these patents disclose photographic elements containing at least one tabular grain emulsion layer the vehicle of which contains a latex. R-3 requires precipitation of the tabular grains in the presence of a peptizer continuous phase which can be an acrylate or methacrylate polymer modified by the inclusion of thioether pendant groups. R-11 discloses vehicles particularly adapted for photothermography. R-22 and R-23 disclose the use of "oxidized" (low methionine) gelatin as a peptizer. Otherwise, the emulsion layer vehicles are identical to those taught to be generally useful in preparing silver halide emulsion layers, illustrated by
R-24 Research Disclosure, Vol. 176, January 1978, Item 17643, Section IX. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
The recent tabular grain emulsions have been observed to provide a large variety of photographic advantages, including, but not limited to, improved speed-granularity relationships, increased image sharpness, a capability for more rapid processing, increased covering power, reduced covering power loss at higher levels of forehardening, higher gamma for a given level of grain size dispersity, less image variance as a function of processing time and/or temperature variances, higher separations of blue and minus blue speeds, the capability of optimizing light transmission or reflectance as a function of grain thickness, and reduced susceptibility to background radiation damage in very high speed emulsions.
While the recent tabular grain emulsions have advanced the state of the art in almost every grain related parameter of significance in silver halide photography, one area of concern has been the susceptibility of tabular grain emulsions to vary in their photographic response as a function of the application of localized pressure on the grains. As might be intuitively predicted from the high proportion of less compact grain geometries in the recent tabular grain emulsions, pressure (e.g., kinking, bending, or localized stress) desensitization, a long standing concern in silver halide photography, is a continuing concern in photographic elements containing recent tabular grain emulsions.
It was recognized prior to the discovery of recent tabular grain emulsions that latices in general when incorporated into silver halide emulsion layers can contribute to reducing pressure desensitization. This teaching is illustrated by
R-25 Research Disclosure, Vol. 195, July 1980, Item 19551.